Field of the Invention
The present invention relates to a planar heating element for a window and a window for a vehicle.
Description of the Background Art
A defogger for removing mist or frost from a window is conventionally attached to the surface of a window of an automobile. Particularly as a defogger for attachment to the surface of a rear window of an automobile, a heating wire-type defogger formed of conductive wires as a heating element is used. An electric heating window pane, which is configured to generate heat when a current is passed through electrode wires embedded in a window pane of a vehicle such as an automobile, is also known.
PTD 1 (Japanese Examined Utility Model Publication No. 4-50203), for example, describes a rear window glass for an automobile configured such that the specific resistance of the transparent conductive thin film is varied depending on the place, in order to preferentially demist or defrost a portion that urgently needs to become visible (see, for example, columns 3, 6 and 7 of PTD 1).
FIG. 18 illustrates a schematic plan view of the conventional rear window glass for an automobile described in PTD 1. The rear window glass for an automobile described in PTD 1 has a structure in which each of opposite sides of a transparent conductive thin film 100 serving as a heating element is provided with an electrode 104 for supplying a current to the transparent conductive thin film 100. In this structure, an upper portion 101 and a lower portion 103 of the transparent conductive thin film 100 have a low specific resistance, and an intermediate portion 102 between the upper portion 101 and the lower portion 103 has a high specific resistance. According to PTD 1, when the rear window glass for an automobile described in PTD 1 having this structure was misted with a spray of stream and then a current was supplied to the transparent conductive thin film 100, the rear window glass was demisted in a short time, with the intermediate portion 102 having a high specific resistance of the transparent conductive thin film 100 being the center (see, for example, column 7 of PTD 1).
The rear window glass for an automobile described in PTD 1 is fabricated as follows (see, for example, columns 6 and 7 of PTD 1). First, a transparent substrate having the shape of a rear window glass for an automobile is washed well with an organic solvent and pure water. Then, a silver paste containing a glass frit as the electrode 104 is screen-printed on each of the upper and lower sides of the transparent substrate, dried and then baked.
Next, the entire transparent substrate is placed in a sputtering vacuum chamber, where the transparent conductive thin film 100 is formed on the surface of the transparent substrate by sputtering. Next, laser beam heating is conducted such that the heating temperature for the intermediate portion 102 of the transparent conductive thin film 100 becomes lower than the heating temperature for the upper portion 101 and the lower portion 103. Consequently, the rear window glass for an automobile described in PTD 1 in which the specific resistance of the intermediate portion 102 of the transparent conductive thin film 100 is higher than the specific resistance of the upper portion 101 and the lower portion 103 is fabricated.